Yesterday I read a fascinating article called "The Death of Llew" by Mike Nichols (of The Witches' Sabbats fame), and it postulated an interesting theory:

So if births are associated with the solstices, when do the symbolic deaths occur? [...] When does darkness conquer light or light conquer darkness? Obviously (to me, at least), it must be at the two equinoxes. At the autumnal equinox, the hours of light in the day are eclipsed by the hours of darkness. At the vernal equinox, the process is reversed. Also, the autumnal equinox, called “Harvest Home”, is already associated with sacrifice, principally that of the spirit of grain or vegetation. In this case, the God of Light would be identical.

[...]

So Midsummer (to me, at least) is a celebration of the Sun God at his zenith, a crowned king on his throne. He is at the height of his power and still one quarter of a year away from his ritual death at the hands of his rival. However, at the very moment of his greatest strength, his dark twin, the seed of his destruction, is born—just as the days begin to shorten.

This made much sense to me, and while it may not be strictly Traditional when it comes to Wicca's understanding of the Wheel, it is a theory I plan to ponder in enriching my own personal practice.